Cleaning rolled magnesium articles



' sary.

Patented Nov. 24., 19 42 ume STATES cmANINe no PATENT OFFICE 2,302,939 I ED MAGNESIUM ARTICLES Herbert 1;. De Long, Midland, Mich, assignor to The Dow Chemical. Company, Midland, Mich, a corporation of Michigan No Drawing;

. 'Ciaims.

This invention relates to an improved method I of removing oxide mill scale from the surfaces of rolled articles of magnesium and magnesiumbase alloys. I s

In fabricating magnesium and magnesium- "base alloys by rolling, the rolled articles frequently tend to pick up particles of mill scale, which usually appear as scattered specks or impe'rfections in the rolled surface. This scale, which consists chiefly of magnesium oxide and its hydrates, finely-divided magnesium metal, and'ca'rbon or carbonized oily lubricant, and will for present purposes be termed "oxide scale," adheres tenaciouslyto the base metal, and cannot is Known chemical methods of removing the ox- I ide mill scale generally involve pickling the rolled magnesium article in a solution of an inorganic move the acid solution, and is then ready for acid, such as hydrochloric, nitric, or sulfuric acid, or mixtures thereof with chromic acid. Unfortunately, these methods, while of some value, are disadvantageous in that the acid attacks the base. metalmore rapidly than it does the oxide scale. As a result, .the scale is not entirely removed unless the time of pickling is prolonged,

with consequent-great loss of metal, and in any case the scaled portions of the surface, after cleaning, appear as raised areas on the pickled V article, imparting a mottled appearance thereto. 5 w ch' a p t ve fi h is applied to Moreover, the pickling acids mentioned tend in use to become inhibited and lose their activity, so that frequent expensive replacement is neces- In view the present invention to provide a simple, effective and inexpensive method of removing oxide mill scale from the surfaces of rolled articlesof magnesium and magnesium-base alloys.

Application January 17 1941, Serial No. 374,894

carboxylic acids, such as formic, acetic, and propionic acids, and hydroxy aliphatic carboxylic.

acids, such as lactic, citric, and'tartaric acids. Acetic, citric, and tartaric acids are articularly 5 effective. The proportion of acid used in the aqueous cleaning solution may range from 0.2 to 50 per cent by weight or more, although concentrations of 5 to per cent are most satisfactory; The cleaning process is customarily car- 10 ried out at room temperature, althoughother temperatures up to the boiling point of the solu 'tion may be used.

In practice, the magnesium or magnesiumis first degreased, if necessary, and is then immersed-in or otherwise subjected to the action of the carboxylic acid solution for a time sufficient to remove the scale, but insufiicient to permit any substantial attack on the metal.

20 Prolonged immersion is to be avoided. In general, at 'room temperature, immersion times of.

0.2 to 1.0 minute are adequate, seconds being especially satisfactory with, acetic and citricacids. After cleaning, the metal is rinsed to reuse.

' The process of the invention is especially advantageous in removing mill scale from rolled articles formed of those magnesium-base alloys 30 which contain a small proportion of manganese as the predominating alloying element, e.g. alloys consisting of 98.5 per cent magnesium and about .1.5 per cent manganese.

It" has alsobeen found that in processes in of these difliculties, it is an object. of 40 form thereon a corrosion-resistant coating, a

pretreatment of the article by immersing it in an aqueous solution of an organic carboxylic acid isdesirable. When the pretreatment is carried out; particularly by immersion in an aqueous so- The invention is based upon the discovery that lution of oxalic acid, an aliphatic carboxylic acid,

aqueous solutions of water-soluble organic carboxylic acids, unlike those of inorganic acids,

attack oxide mill 'scale much more rapidly than they do magnesium metal. Accordingly, by using such acids, oxide scale can be effectively removed without damage'to the base metal. Moreover, the acids have a long effective life, and need bereplaced only at infrequent intervals.

5;..f I The carboxylic acids most suitable in the mocess of the inventionare oxalic acid, aliphatic such as acetic acid, or a hydroxy aliphatic carbase alloy articles to be cleaned of oxide scale itative of the invention. 7

- loys which comprises immersing the aqueous solution of a carboxylic acid selected scription is illustrative rather than strictly lim- I claim: A

1.-A method of removing carbonized oil containing oxide mill scale from the surfaces of rolled articles of magnesium and magnesium-base alloys which comprises subjecting the article to the action of an aqueous solution of a water-soluble organic carboxylic acid for a time sufllcient to remove the scale but insufficient to permit any substantial attack on the metal.

2. A method of removing carbonized oil con- I taining oxide mill scale from the surfaces ofrolled articles of magnesium and magnesium-base alfrom the class consisting of oxalic acid, aliphatic carboxylic acids, .and' hydroxy aliphatic carboxylic acids for a time sufflcient to remove the scale 'but insufiicient to permit any substantial attack on the metal.

3. A method according to claim 2 wherein the acid is acetic acid.

- 4. A method according to claim 2 wherein the acid is citric acid.

5. A method according t0 c1aim 2 wherein the acid is tartaric acid. I

. 6. A method of removing carbonized oil containing oxide mill scale from the surfaces of rolledarticles of magnesium-base alloys containing a small proportion of manganese as the predominant alloying element which comprises immersarticle in an ing the article in a 5 to 15 per cent by weight' aqueous solution of citric acid at room temperature for 0.2 to 1.0 minute.

8. In a method for producing a protective finish upon the surface of rolled articles of magnesium and magnesium-base alloys having a carbonized oil-containing oxide mill scale on the surface thereof wherein the article is subjected to the action of an aqueous coating solution adapted to react chemically with the metal surface to form thereon a corrosion-resistant coating, the step -which comprises immersing the article in an aqueous solution of a carboxylic acid selected from the class consisting. of oxalic acid, aliphatic carboxylic acids, and hydroxy aliphatic carboxylic acids for a time sufllcient to remove scale but insufllcient to permit any substantial attack on the metal, priorto subjecting the article to th action of the coating solution. v

9. A method according to claim 8 wherein the carboxylic acid is acetic acid.

10. A method according to claim 8 wherein the carboxylic acid is citricacid.

. HERBERT K. DE LONG. 

